BACKGROUND While methadone effectively treats opiate dependence, the side effect of erectile dysfunction (ED) may interfere with treatment adherence and benefits. OBJECTIVE To determine the rate of ED and the associated factors which predict ED in male patients on methadone maintenance therapy (MMT) in a Malaysian population. METHODS The main outcome measures were the International Index of Erectile Function-15 (IIEF-15) and the Beck Depression Inventory (BDI). METHODS A total of 108 participants diagnosed with heroin dependence were assessed. We used the Structured Clinical Interview for DSM-IV Axis-I Disorders (SCID-I) on subjects who received MMT, and they were assessed using the IIEF-15, the BDI, and measures of other clinical and sociodemographic variables. RESULTS The rate of ED among men on MMT was 68.5% (mild ED, 36.1%; mild to moderate ED, 22.2%; severe ED, 3.7%). The mean age of the participants was 43.45 years. Older age (P = 0.002), concurrent illicit heroin use (P = 0.024), and having an older partner (P = 0.039) were significantly associated with ED. Following multivariate analysis, it was found that older age was the only significant predictor of ED, with an adjusted odds ratio of 1.07 (95% CI = 1.02-1.16). Methadone dose and duration of methadone treatment were not significantly associated with ED. CONCLUSIONS ED was highly prevalent among male patients on MMT. This suggests that there is a need for routine assessment of sexual function in patients on methadone. Among the risk factors, age was the only factor that was significantly associated with ED. The current use of MMT in Malaysia in terms of dosage and duration did not pose a significant risk for ED.